This invention relates to metal oxide compounds and to preparation methods thereof. More specifically, this invention relates to doped metal oxide insertion compounds for use in lithium and lithium-ion batteries.
Metal oxides such as lithium metal oxides have found utility in various applications. For example, lithium metal oxides have been used as cathode materials in lithium secondary batteries. Lithium and lithium ion batteries can be used for large power applications such as for electric vehicles. In this specific application, lithium or lithium ion cells are put in series to form a module. In the event that one or more of the cells in the module fails, the rest of the cells become overcharged resulting possibly in explosion of the cells. Therefore, it is important that each cell is individually monitored and protected against overcharging.
The most attractive materials for use as cathode materials for lithium ion secondary batteries have been LiCoO2, LiNiO2, and LiMn2O4. However, although these cathode materials are attractive for use in lithium ion secondary batteries, there are definite drawbacks associated with these materials. One of the apparent benefits for using LiNiO2 and LiCoO2 as cathode materials is that those lithium metal oxides have a theoretical capacity of 275 mAxc2x7hr/g. Nevertheless, the full capacity of these materials cannot be achieved in practice. In fact, for pure LiNio2 and LiCoO2, only about 140-150 mAxc2x7hr/g can be used. The further removal of lithium by further charging (overcharging) the LiNiO2 and LiCoO2 material degrades the cycleability of these materials by moving nickel or cobalt into the lithium layers. Furthermore, the further removal of lithium causes exothermic decomposition of the oxide in contact with the organic electrolyte under heated conditions which poses safety hazards. Therefore, lithium ion cells using LiCoO2 or LiNiO2 are typically overcharge protected.
LiCoO2 and LiNiO2 have additional disadvantages when used in lithium ion batteries. Specifically, LiNiO2 raises safety concerns because it has a sharper exothermic reaction at a lower temperature than LiCoO2. As a result, the charged end product, NiO2, is unstable and can undergo an exothermic decomposition reaction releasing O2 (Dahn et al, Solid State Ionics, Vol. 69, 265 (1994)). Accordingly, pure LiNiO2 is generally not selected for use in commercial lithium-ion batteries. Additionally, cobalt is a relatively rare and expensive transition metal, which makes the positive electrode expensive.
Unlike LiCoO2 and LiNiO2, LiMn2O4 spinel is believed to be overcharge safe and is a desirable cathode material for that reason. Nevertheless, although cycling over the full capacity range for pure LiMn2O4 can be done safely, the specific capacity of LiMn2O4 is low. Specifically, the theoretical capacity of LiMn2O4 is only 148 mAxc2x7hr/g and typically no more than about 115-120 mAxc2x7hr/g can be obtained with good cycleability. The orthorhombic LiMnO2 and the tetragonally distorted spinel Li2Mn2O4 have the potential for larger capacities than is obtained with the LiMn2O4 spinel. However, cycling over the full capacity range for LiMnO2 and Li2Mn2O4 results in a rapid capacity fade.
Various attempts have been made to either improve the specific capacity or safety of the lithium metal oxides used in secondary lithium batteries. For example, in an attempt to improve the safety and/or specific capacity of these lithium metal oxides, these lithium metal oxides have been doped with other cations. For example, lithium and cobalt cations have been used in combination in lithium metal oxides. Nevertheless, although the resulting solid solution LiNi1xe2x88x92XCoXO2 (0xe2x89xa6Xxe2x89xa61) may have somewhat improved safety characteristics over LiNiO2 and larger useful capacity below 4.2 V versus Li than LiCoO2, this solid solution still has to be overcharge protected just as LiCoO2 and LiNiO2.
One alternative has been to dope LiNiO2 with ions that have no remaining valence electrons thereby forcing the material into an insulator state at a certain point of charge, and therefore protecting the material from overcharge. For example, Ohzuku et al (Journal of Electrochemical Soc., Vol. 142, 4033 (1995)) describe that the use of Al3+ as a dopant for lithium nickelates (LiNi0.75Al0.25O4) can produce improved overcharge protection and thermal stability in the fully charged state as compared to LiNiO2. However, the cycle life performance of this material is unknown. Alternatively, U.S. Pat. No. 5,595,842 to Nakare et al. demonstrates the use of Ga3+ instead of Al3+. In another example, Davidson et al (U.S. Pat. No. 5,370,949) demonstrates that introducing chromium cations into LiMnO2 can produce a tetragonally distorted spinel type of structure which is air stable and has good reversibility on cycling in lithium cells.
Although doping lithium metal oxides with single dopants has been successful in improving these materials, the choice of single dopants which can be used to replace the metal in the lithium metal oxide is limited by many factors. For example, the dopant ion has to have the right electron configuration in addition to having the right valency. For example, Co3+, Al3+, and Ga3+ all have the same valency but Co3+ can be oxidized to Co4+ while Al3+, and Ga3+ cannot. Therefore doping LiNiO2 with Al or Ga can produce overcharge protection while doping with cobalt does not have the same effect. The dopant ions also have to reside at the correct sites in the structure. Rossen et al (Solid State Ionics Vol. 57, 311 (1992)) shows that introducing Mn into LiNiO2 promotes cation mixing and therefore has a detrimental effect on performance. Furthermore, one has to consider the ease at which the doping reaction can be carried out, the cost of the dopants, and the toxicity of the dopants. All of these factors further limit the choice of single dopants.
The present invention uses multiple dopants to replace the transition metal M in lithium metal oxides and metal oxides having the formula LiMyOz or MyOz to have a collective effect on these intercalation compounds. As a result, the choice of dopants is not limited to elements having the same valency or site preference in the structure as the transition metal M, to elements having only a desired electron configuration, and to elements having the ability to diffuse into LiMyOz or MyOz under practical conditions. The use of a carefully chosen combination of multiple dopants widens the choices of dopants which can be used in the intercalation compounds and also can bring about more beneficial effects than a single dopant. For example, the use of multiple dopants can result in better specific capacity, cycleability, stability, handling properties and/or cost than has been achieved in single dopant metal oxides. The doped intercalation compounds of the invention can be used as cathode materials in electrochemical cells for lithium and lithium-ion batteries.
The doped lithium metal oxides and doped metal oxides of the invention have the formula:
LiMyxe2x88x92x[A]xOz or Myxe2x88x92x[A]xOz, 
wherein M is a transition metal, 0 less than xxe2x89xa6y, [A]=      ∑          i      =      1        n    ⁢            w      i        ⁢          B      i      
wherein Bi is an element used to replace the transition metal M and wi is the fractional amount of element Bi in the total dopant combination such that                     ∑                  i          =          1                n            ⁢              w        i              =    1    ,
n is the total number of dopant elements used and is a positive integer of two or more, the fractional amount wi of dopant element Bi is determined by the relationship                     ∑                  i          =          1                n            ⁢                        w          i                ⁢                  E          i                      =                  the        ⁢                  xe2x80x83                ⁢        oxidation        ⁢                  xe2x80x83                ⁢        state        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        the        ⁢                  xe2x80x83                ⁢        replaced        ⁢                  xe2x80x83                ⁢        transition        ⁢                  xe2x80x83                ⁢        metal        ⁢                  xe2x80x83                ⁢        M            ±      0.5        ,
Ei is the oxidation state of dopant Bi in the final product LiMyxe2x88x92x[A]xOz or Myxe2x88x92x[A]xOz, the dopant elements Bi are cations in the intercalation compound, and the ratio of Li to 0 in the doped intercalation compound is not smaller than the ratio of Li to O in the undoped compound LiMyOz or MyOz. Typically, M is selected from Co, Ni, Mn, Ti, Fe, V and Mo and the dopant elements Bi are any elements other than M having a Pauling""s electronegativity not greater than 2.05 or Mo.
In one preferred embodiment of the invention, the intercalation compound has a formula LiMyxe2x88x92x[A]xOz wherein M is Ni or Co and the dopant elements Bi include Ti4+ and Mg2+. The formulas LiNi1xe2x88x92xTiaMgbO2 and LiCo1xe2x88x92xTiaMgbO2 can also be used to describe these intercalation compounds wherein x=a+b and x is preferably in the range from greater than 0 to about 0.5. More preferably, a is approximately equal to b and b is no smaller than a for these intercalation compounds. The dopant elements Bi can further include other cations or have the formula LiMyxe2x88x92x[A]xOz wherein M is Ni or Co, y=1, z=2, and the dopant elements Bi include Ti4+, Mg2+ and Li+ cations.
The present invention also includes a method of preparing a doped intercalation compound having the formula LiMyxe2x88x92x[A]xOz or Myxxe2x88x92y[A]xOz. Source compounds containing M, [A] and optionally Li are mixed to provide a stoichiometric relationship between M, [A] and Li corresponding to the formula LiMyxe2x88x92x[A]xOz or Myxe2x88x92x[A]xOz, wherein M is a transition metal, 0 less than xxe2x89xa6y,       [    A    ]    =            ∑              i        =        1            n        ⁢                  w        i            ⁢              B        i            
wherein Bi is an element used to replace the transition metal M and wi is the fractional amount of element Bi in the total dopant combination, n is the total number of dopant elements used and is a positive integer of two or more, the fractional amount wi of dopant element Bi is determined by the relationship:                     ∑                  i          =          1                n            ⁢                        w          i                ⁢                  E          i                      =                  the        ⁢                  xe2x80x83                ⁢        oxidation        ⁢                  xe2x80x83                ⁢        state        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        the        ⁢                  xe2x80x83                ⁢        replaced        ⁢                  xe2x80x83                ⁢        transition        ⁢                  xe2x80x83                ⁢        metal        ⁢                  xe2x80x83                ⁢        M            ±      0.5        ,
Ei is the oxidation state of dopant Bi in the final product LiMyxe2x88x92x[A]xOz or Myxe2x88x92z[A]xOz, the dopant elements Bi are selected to be cations in the intercalation compound, and the ratio of Li to O in the doped intercalation compound is not smaller than the ratio of Li to O in the undoped compound LiM4P2 or MyOz. The cations for the intercalation compound can each be supplied from separate source compounds or two or more of the cations can be supplied from the same source compounds. The mixture of source compounds is fired at a temperature between 500xc2x0 C. and 1000xc2x0 C. in the presence of oxygen to produce the intercalation compound and preferably cooled in a controlled manner to produce a doped intercalation compound suitable for use as a cathode material for electrochemical cells for lithium and lithium-ion batteries.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description which describes both the preferred and alternative embodiments of the present invention.